Method and apparatus for preparation of aluminium coagulating agent

ABSTRACT

A coagulating agent for use in water treatment is prepared by subjecting a solution to electrolysis between electrodes, at least one of which consists of aluminum as the anode. The solution contains a strongly electrolytic salt and a strong acid, in which aluminum dissolves by electrolysis.

United States Patent Yokozeki et al.

Feb. 6, 1973 METHOD AND APPARATUS FOR PREPARATION OF ALUMINIUM COAGULATING AGENT Inventors: Zenzo Yokozeki; Takeshi Hatta,

both of Yokohama-shi, Japan 6 PH OF SOLUTION [56] References Cited UNITED STATES PATENTS 1,382,808 6/1921 Sem ..204/96 2,392,531 1/1946 Huehn 1 "204/96 3,240,687 3/1966 Konig et a1. ..204/94 Primary ExaminerHelen M. McCarthy Assistant Examiner-H. A. Feeley Attorney-Woodhams, Blanchard and Flynn [5 7] ABSTRACT A coagulating agent for use in water treatment is prepared by subjecting a solution to electrolysis between electrodes, at least one of which consists of aluminum as the anode. The solution contains a strongly electrolytic salt and a strong acid, in which aluminum dissolves by electrolysis.

5 Claims, 10 Drawing Figures PATENTEUFEB ems 3,715,290

SHEET u 0F 4 FIG. I0

ORNEYS METHOD AND APPARATUS FOR PREPARATION OF ALUMINIUM COAGULATING AGENT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of preparing a coagulating agent by effecting electrolysis employing an aluminum anode and thereby causing aluminum to dissolve into the electrolyte and an apparatus pertaining to said method.

2. Description of the Prior Art It has hitherto been proposed to utilize a solution containing aluminum, which is prepared by electrolysis employing an aluminum anode, as a coagulating agent for impure water. But, because of the incidental formation of scale and the deficient coagulatability of the resultant coagulating agent, these proposals have so far been of little practical use. There has also been proposed a method of electrolysis employing a diaphragm provided between the electrodes, but this method has required complicated operations in order to prevent the occurence of clogging of the meshes of the diaphragm to control constantly the electrolyte flux in the two chambers partitioned by said diaphragm, and therefore has not been advisable.

SUMMARY OF THE INVENTION One object of the present invention is to overcome the foregoing drawbacks of the prior art and to provide a method of preparing efficiently a coagulating agent having a high coagulatability as well as an apparatus relevant thereto, which comprises subjecting a mixed solution of a strongly electrolytic salt and a strong acid to electrolysis by employing aluminum as the anode and thereby causing aluminum to dissolve into the electrolyte.

Another object of the present invention is to prevent the formation of scale by disposing aluminum anodes on both sides of the cathode.

Still another object of the present invention is to effect enlargement of the apparatus by dividing the electrolyzer with partitions into a plurality of electrolytic chambers provided with, for each unit electrodes comprising the anodes and the cathode.

A further object of the present invention is to improve the means of installing aluminum anode plates in the electrolyzer, thereby to simplify the manufacture of the electrode plates and the process of installation thereof.

A still further object of the present invention is to provide a fixed concentration of aluminum solution by controlling the level of each solution within the electrolyte tank and the electrolyzer.

A strongly electrolytic salt is employed for the electrolyte in the present invention to maintain the conductivity of the electrolyte. The applicable salts include, for instance, sodium chloride, sodium sulfate, etc., and the concentration thereof is to be in the range of 0.1-0.5 wt%. As the strong acid, hydrochloric acid, sulfuric acid, etc. is applicable, and its concentration is in the range of 0.05-l wt%.

As for the electrode, an aluminum plate is employed as the anode, while, as the cathode, general electroconductive materials such as aluminum, nickel, lead, carbon, etc. are applicable. Electrolysis is effected by means of disposing said anode and cathode at spacings of 10-40mm within the electrolyzer and applying thereto an electric current having a voltage of 2-30V and current density of 1-2 A/dm. In case of electrolysis employing, as an electrolyte, a mixed solution of sodium chloride and hydrochloric acid, hydrogen gas is produced at the cathode by electric discharge of H, while aluminum ion is discharge from the anode. At the initial stage of electrolysis, due to existence of a large amount of hydrochloric acid within the electrolyte, the dissolved aluminum ion turns into ALCL but, with the progress of electrolysis, the entire hydrochloric acid is consumed as ALCL And, as the electrolysis is further continued thereafter, hydrogen gas produced by decomposition of water is given off from the cathode, while aluminum is dissolved as polyaluminum from the anode. The polyaluminum is considered to be a product of the polynuclear coordination of Al and OH. And, this substance does not become a precipitate but remains in the state of solution. The polyaluminum thus dissolved in the solution is possessed of a unique structure and, as a result, is excellent in coagulating efficiency. A conventional polyaluminum is prepared by gradually adding a strong alkali such as calcium hydroxide to AlCl solution, and, as a result, there exist anions (Cl') equivalent to aluminum in the solution. Said anion is detrimental to coagulation. According to the present invention, however, the amount of aluminum to be dissolved exceeds the equivalent of hydrochloric acid and sodium chloride added to the electrolyte, and, therefore, the coexisting anions are extremely reduced. Addition of sodium chloride to the electrolyte is for the purpose of maintaining the conductivity of the electrolyte. That is, in case of omitting the addition of sodium chloride, the conductivity of the electrolyte decreases with the progress of electrolysis and the consumption of hydrochloric acid, but, in case where sodium chloride is added, said decrease of conductivity is prevented.

The results of the tests of the method of the present invention compared with that of the prior art are as shown in the following.

By employing three kinds of electrolytes comprising (1) 0.05 N-hydrochloric acid, (2) 0.05 N-sodium chloride and (3) 0.05 N-hydrochloric acid and 0.05 N- sodium chloride, respectively and two sheets of aluminum plates measuring 30mm 30mm each as electrodes, electrolysis was carried out respectively disposing the plates at intervals of 10mm, the amount of electrolyte being 200 m1 and the current density being 20 mA/cm. In this case, the changes of voltage were as shown in the following Table 1.

TABLE 1 Period of electrolysis (min.) 0 20 60 1 volts 0.7 2 ll 27 2 volts 3 3 3 3 3 volts 4 4 4 4 As seen in the foregoing table 1, in case of the electrolyte l employing hydrochloric acid alone, continuous electrolysis became impossible due to a rapid increase of the liquid resistance. In case of the electrolyte 2 employing sodium chloride alone, though there was observed no change of the liquid resistance, the aluminum was precipitated as a hydroxide and was unusable as a coagulating agent. In case of the electrolyte 3 employing the foregoing mixture, the liquid resistance did not change, the dissolved aluminum was perfectly soluble and the electrolyte was transparent. The coagulating agent thus prepared is possessed of a high coagulatability, which agent is added to an impure water or colored water to coagulate, flocculate and precipitate suspended matters therefrom. The amount of this coagulating agent to be added to an impure water is in the range of 1-5 ppm in terms of Al. The coagulating agent is employed in the same way as the conventional coagulation and flocculation method in which pH-control as well as rapid agitation and slow agitation are effected to form flocks which are precipitated or separated by filtering.

A coagulating agent produced by electrolysis under the present invention is possessed of a high coagulatability, and, not only that, the method of preparation thereof is so simple that it may be prepared at the place of use by a simple process. Electrolysis can be effected at a constant voltage and, accordingly, a stable production of coagulating agent can be expected. Further, due to the employment of an acid as the electrolyte, the formation of scale onto the cathode is lessened. There being no necessity for providing any diaphragm for the electrolyzer, it is capable of effecting electrolysis at a low voltage by lessening the interval distance between electrodes and without fear of any rise in voltage due to clogging of the meshes of the diaphragm. Preparation of said coagulating agent may be performed through either electrolysis of a batch type or that of a continuous system.

According to the present invention, although the formation of scale onto the cathode is comparatively small because of the employment of the acid as the electrolyte, a lot of OI-I'is coordinated with aluminum in the electrolyte, resulting in a high basicity of the electrolyte, and, accordingly, the aluminum ion is in such a state that it may be easily separated. Therefore, employment of an electrolyzer of the same type as that of the prior art, such as an electrolyzer provided with aluminum electrodes alone, would bring about adhesion of scale onto the cathode, and is not appropriate for the purpose of a long-term operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the results of tests on the ability of various compounds to reduce the color of solutions, as described in Example 1.

FIGS. 2, 3, 4 and 6 show cross-sections of electrolyzers employing conductive materials other than aluminum as the cathode, said electrolyzers shown in FIGS. 2-4 being ones designed for long-term operation while the electrolyzer shown in FIG. 6 being one designed for mass production.

FIG. shows the state of the electric current withi the electrolyzer of FIG. 3.

FIG. 7 is an oblique view illustrating the manner of retaining the partition employed for the electrolyzer of FIG. 6.

FIG. 8 is an oblique view of an electrode fixture designed to improve the one employed for the conventional electrolyzer.

FIG. 9 is an oblique view of the electrode fixture for a conventional electrolyzer.

FIG. 10 is a flow sheet showing an example of the electrolytic system for preparing a solution of a fixed concentration of a coagulating agent.

Suitable electrolyzers to be employed in the present invention are described below and are illustrated in the accompanying drawings.

In FIGS. 2-4, reference numeral 1 denotes an aluminum anode plate, 2 denotes an iron cathode plate and 3 denotes an electrolyte. The electrolyzer shown in FIG. 2 is the minimum size of the electrolyzer suitable for long-term operation, and its cathode 2 consists of an iron plate, while on both sides of said cathode there are disposed aluminum anodes 1. The electrolyzers shown in FIGS. 3 and 4 are modifications of the one shown in FIG. 2, and respectively contain a multiplicity of electrodes disposed therein. Either one of said modifications is so designed that an aluminum anode plate is disposed on both sides of the cathode plate 2. Further, in FIG. 3,@denotes the conductor expressing the state of connection between the electrodes, which functions to effect electrolysis by impressing voltage as shown with marksGBandG. In order to prepare a coagulating agent, electrolysis is effected by applying an electric current for a fixed period of time, and the aluminum solution prepared by the electrolysis is taken out and used as coagulating agent. In case said aluminum anode plate is completely consumed after repeated electrolyses, new anode plates are provided on both sides of the cathode plate and an electric current is again applied thereto, whereby electrolysis is resumed. The cathode plate is not consumed and is usable continuously. The electrolyte may be supplied either a batch type feed wherein said electrolyte is renewed for each cycle of electrolysis or by continuous system wherein the electrolyte is flowed in continously. As for the cathode plate for electrolyzer intended for a long-term operation, any conductive material other than aluminum, for instance, iron, nickel, carbon, lead, etc., is applicable.

In case of an electrolyzer intended for a long-term operation, due to the disposition of anode plates on both sides of each cathode plate thereof, a uniform current density is maintained even on the back side of cathode plate, the formation of scale is inhibited, and a long-term operation thereof is possible. In other words, unlike those electrolyzers provided with aluminum electrodes alone or having the anode and the cathode disposed alternately, there is no fear of theoperation being interrupted by adhesion of scale onto the cathode nor the necessity for changing the cathode, and, as a result, the management of the operation can be improved and an aluminum solution of excellent coagulatability can be steadily produced for a long time.

A mass production of the aluminum coagulating agent by the method of the present invention naturally requires a large-sized electrolyzer and a multiplicity of aluminum anode plates. In this case, from the view point of efficiency, a method of electrolysis through series connection of electrode units wherein the aluminum anodes are disposed on both sides of the cathode is applied. In case of a series connection as above, a major portion of the current runs in the direction indicated by the solid-line arrow shown in FIG. 5 (wherein l-a, 1a', I-b and l-b' denote respectively aluminum anode plate, 2 and 2' denote respectively iron cathode plate and 3 denotes electrolyte), while a portion thereof leaks also in the direction indicated by the dotted-line arrow, resulting in discrepancy in the solubility of each aluminum plate and making it difficult to effect a uniform dissolution of aluminum, and, accordingly, the scale tends to adhere onto the anode plate l-b adjacent to the cathode plate 2'. To avoid this tendency, a method wherein a plurality of aluminum anode plates are placed in order as independent electrolyzers so as to effect electrolysis by means of electrical series connection thereof may be adopted. But, it is extremely uneconomical to provide so many independent electrolyzers. For the purpose of mass production, it is therefore advisable to employ electrolyzer, the interior of which is divided by simple partitions into a plurality of electrolytic chambers arranged in a row so as to permit the interflow of electrolyte therein while causing almost no mutual interference electrically and said electrolytic chambers are provided with electrode units which are electrically connected in series.

More specifically, the interior of the electrolyzer is divided by partitions of larger size than the electrode plates into a plurality of electrolytic chambers interconnected with each other, each electrolytic chamber is provided with an electrode unit consisting of aluminum electrodes disposed on both sides of a cathode plate, and all the electrode units are mutually connected in series.

Now, an explanation of the'foregoing electrolyzer for use in mass production will be given in the following by reference to FIG. 6.

In FIG. 6, l-a, l-a, l-b, l-b', l-c and l-c' respectively denote the aluminum anode plates, 2, 2' and 2" respectively denote the iron cathode plates, 3 denotes the electrolyte, 4 and 4' respectively denote the partitions and A, B and C respectively denote the unit electrolytic chambers.

For the partitions denoted by 4 and 4', an electric insulating material such as polyvinyl chloride is suitable. Said partition is required to be larger than the electrode plate in order to make each unit electrolytic chamber practically free from electrical interference. The size required for the partition is more than about 1.5 times as large as the electrode plate in area in case where the lengths and the breadths of the anode and the cathode are respectively equal (but the length and the breadth of the partition are larger than the corresponding sides of the electrode plate).

Therefore, it is not necessary to design and manufacture the partition according to the size of the electrolyzer so as to completely isolate each unit electrolytic chamber by said partition, but it suffices to make the size of partition somewhat smaller than the electrolyzer and, therefore, it can be easily manufactured.

As to the means of fitting the partition, it suffices to fit the electrode retainer 5 with partitions 4 and 4' and hang the resultant integral unit in the electrolyzer as illustrated in FIG. 7. 6 denotes the grooves for retaining the electrode plates, and, by placing the partitions 4 and 4 at intervals spaced apart by an appropriate numbers of said grooves 6, each electrode and the partitions 4 and 4' can be retained 'at a fixed distance.

Meanwhile, a desirable means of fitting the electrode plate is as shown in FIG. 8. In said FIG. 8, 7 denotes the top edge of the electrode plate, 8 denotes the body of the electrode plate, 9 denotes the feeding terminal provided on one or two spots on the top edge of said electrode plate proper 8, 10 denotes the hook-shape handle provided on the respective upper ends of the two side edge of the electrode plate proper 8, l1 denotes the retainer for supporting the electrode plate 7 at the position of said handle 10, and 12 denotes the guide for preventing the electrode plates from coming into contact with each other. The electrode plate proper 8 is made of aluminum plate in case of its being employed as the anode, while, in case of its being employed as the cathode, it is made of aluminum, iron, nickel, lead, carbon and the like. The feeding terminal 9 and the handle 10 are made of such a metal as aluminum, copper and iron. Preparation of said electrode plate comprises the steps in which: a material such as an aluminum plate trimmed to a prescribed size is employed as the electrode plate proper; the spot on said plate corresponding to the feeding terminal 9 as shown in FIG. 8 is tapped; a wire-connected feeding terminal is screwed down by way of said tapped hole; and the hook-shape handle is put on the respective end the edges adjacent to the feeding terminals by boring a hole or tapping and fixing it with a binding agent or screws. It is to be noted that, though the handle 10 is described as hook-shaped in the present invention, it includes handles of every shape that may be suitable for lifting the plate by holding with fingers and the like. As for the retainer, it is prepared by employing an appropriate, electrically insulating material such as wood, plastic, etc. and engraving thereon a plurality of grooves having a width corresponding to the thickness of the electrode plate, as the guide.

For reference, a conventional fitting apparatus of this category is shown in FIG. 9. In said FIG. 9, the conventional apparatus comprises the electrode plate body 8' the feeding terminal 9' provided on one or two spots of the upper edge 7 of said electrode plate and the handles 10' provided on both ends of the upper edge, and the electrode plate 8 is held by the retainer 11 in the electrolyzer by means of the portion provided as the handle 10'. Said feeding terminal 9 and handle 10' are fixed by welding a piece of aluminum having an appropriate size onto the electrode plate body 8 and, accordingly, the electrode plate not only requires a considerable processing cost, but also is inconvenient for handling in case of changing the electrode. Further, inasmuch as the aluminum anode plate is dissolved into the electrolyte as aluminum ions by electrolysis and is consumed usually in about 1 week to 1 month, it has been considered unwise to incur the enormous processing cost for such expendables, and, at the same cupying the minimum space has a merit of preventing electrode plates from coming into contact with each other.

Next, another preferred electrolyzer to be employed for the method of the present invention is described in the following. The electrolyzer for preparing a coagulating agent in this case is of the batch type, that is, it is devised for repeating operations in which electrolyte is supplied for each cycle and, upon electrolysis for a fixed period of time causing the aluminum of the anode to dissolve, the aluminum solution is taken out. Whenever the aluminum anode plate is exhausted, it is renewed and said operation is continued. In case of preparing a coagulating agent by the batch type method as above, it is necessary to ensure obtaining an electrolyzed solution, that is, a solution of coagulating agent, having always a fixed concentration.

However, in case of a conventional electrolyzer, inasmuch as the dissolution of the aluminum anode plate takes place with the progress of electrolysis resulting in a decrease in the volume of said plate and the electrolyte is poured into the electrolyzer up to a fixed level, there occur fluctuations of the volume of the replaced electrolyte with every cycle and, as a result, even though the electrolysis is effected with a fixed amperage, a fixed concentration of a coagulating agent solution can not be obtained.

Therefore, the inventors of the present invention have devised a batch-type electrolytic apparatus capable of producing a coagulating agent solution with a fixed concentration.

To be precise, the present apparatus consists of the electrolyte tank, the electrolyzer wherein the electrolyte is introduced from the electrolyte tank so as to cause dissolution of the aluminum anode by electrolysis therein, the means of controlling the electrolyte being introduced into the electrolyzer at a fixed solution level, the electrolyzed solution tank wherein the elec: trolyzed solution is introduced from the electrolyzer, and the means of introducing the electrolyte remaining in the electrolyte tank as well as the electrolyzed solution of the electrolyzer after finishing the electrolysis, and is so devised as to produce an electrolyzed solution of aluminum having a fixed concentration.

The mechanism of the present apparatus is hereunder described with reference to FIG. 10. To

begin with, a highly concentrated electrolyte stored in the electrolyte storage tank 13 is poured into the measuring tank 16 by means of the pump 14 through the pipe 15. When the quantity of said electrolyte comes to a fixed level in the measuring tank 16, the level switch 17 operates and the pump 14 stops operating. A fixed quantity of the highly concentrated electrolyte thus measured is poured into the electrolyte tank 20 through the pipe 19 by the opening of the valve 18, and simultaneously with completion of pouring of said quantity, the operation valve 21 opens and the diluting water is introduced into the electrolyte tank 20 through the pipe 22 until a prescribed level of liquid is reached. Said level of liquid is controlled by way of closing said valve 21 through operation of the level switch 23. The highly concentrated electrolyte poured into the electrolyte tank 20 is thus diluted down to a prescribed concentration. Next, the diluted electrolyte is poured into the electrolyzer 26 through the pipe 25, by the opening of the operation valve 24, until a fixed level of liquid is reached. The electrolyzer is provided with the level switch 27, and, whenever the quantity of theelectrolyte reaches a prescribed level of liquid, said level switch operates to close the valve 24. The electrolyzer is provided with the aluminum anode and the iron cathode as disposed side by side to see that there remain a small amount of electrolyte within the electrolyte tank at the stage where said level switch operates and the valve 24 is closed. Thus, the electrolysis is effected in the electrolyzer for a fixed period of time with a fixed amperage, whereby the aluminum anode is dissolved and coagulating agent is prepared. In this connection, 1 represents the anode, 2" represents the cathode, 4" represents the partition and 28 represents the rectifier. All the electrolyzed solution pours into the electrolyzed solution tank 31 through the pipe 30 as the operation valve 29 opens. At the same time, the valve 24 belonging to the electrolyte tank 20 opens and the remaining electrolyte in the electrolyte tank flows into the electrolyzed solution tank 31 through the electrolyzer 26 to be mixed with previously prepared electrolyzed solution.

Whenthe electrolysis is thus carried out repeatedly and the volume of the aluminum electrode occupying the electrolyzer gets reduced due to dissolution thereof, the electrolyte is poured into the electrolyzer up to a fixed level, thereby resuming the electrolysis. On this occasion, there naturally occur fluctuations in the quantity of the electrolyte within the electrolyzer. According to the present method, however, the volume of aluminum to be dissolved is regulated and the quantity of the solution to be consumed for every cycle comes to be always fixed (e.g. the volume up to the height of the level switch 23 of the electrolyte tank in FIG. 10) by means of mixing the electrolyzed solution with the residual electrolyte of the electrolyte tank and the electrolyzed solution within the electrolyzed solution tank 31 can be ensured to have a fixed concentration through electrolysis with a fixed amperage.

The electrolyzed solution regulated to have a fixed concentration is stored in the electrolyzed solution tank 31, taken out through the discharge pipe 32 as occasion demands, and served for use as a coagulating agent by means of adding to a water current and the like.

The apparatus of the present invention is not limited to the foregoing example. That is, the highly concentrated electrolyte and the diluting water may be previously fixed up for a prescribed concentration prior to being poured into the electrolyte tank. Besides, it is not always required to provide the level switch 23 for the electrolyte tank 20, and the supply of the electrolyte in a fixed quantity may be performed by means of supplying to the electrolyte tank 20 for a fixed period of time by employing a timer-equipped electromagnetic valve. Further, as to the cathode in the electrolyzer, the aforesaid conductive material such as aluminum, iron, etc. is employed, and it does not matter if the partition is provided or not.

In case where electrolytic dissolution of the aluminum anode plate is effected within an aqueous solution containing a strongly electrolytic salt by employing such electrolytic apparatus, there is obtained a solution of electrolyzed aluminum coagulating agent having a fixed concentration irrespective of the extent of consumption of said aluminum anode plate, and the administration of coagulating agent is extremely simplified. Besides, each valve and the level switch are capable of controlling electrolytically by means of a program timer, and the electrolytic apparatus may also be so devised as to operate automatically.

Accordingly, the present electrolytic apparatus is useful for preparing a solution of aluminum coagulating agent having a fixed concentration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 By employing two sets of electrodes provided within the electrolyzer, each set thereof consisting of four sheets of 30cm X 30cm aluminum plates disposed at intervals of 10mm, of which the outer two plates were connected with a source of electric power, and using a mixture solution of 0.2 percent sodium chloride and 0.2 percent hydrochloric acid as the electrolyte, electrolysis was effected causing the dissolution of aluminum. When said electrolysis was continued for 40 min. by applying a power source of 8.5V, the current value got stabilized where the current value was 23-25A. The dissolution of aluminum resulting from said electrolysis was at the rate as shown in Table 2, and it is observed that there took place an immense effusion of aluminum not in the form of ions when the electrolysis was continued for more than 30 min.

TABLE Period of electrolysis (min.) 30 60 120 Total aluminum (ppm) 630 1200 2400 Ionized-state aluminum (ppm) 420 360 360 The aluminum solution prepared by the electrolysis for 60 min. was subjected to a Jar-test wherein Al solution was added to a colored water to such an extent that the concentration in terms of aluminum would be 237 ppm and the water was subjected to 5 min. rapid agitation and min. slow agitation. The result of said test was as shown in FIG. 1. Said colored water employed on this test was prepared by adding fulvic acid to the tap water and its chromaticity (color) was 50 degrees. For further reference, the results of tests conducted on aluminum sulfate and aluminum chloride under the same conditions as the foregoing are also shown in the drawing. In FIG. 1, the-abscissa represents the pH value of the solution, while the ordinate represents the residual chromaticity (color) of the solution, and it is obvious therefrom that the range of coagulating pH is wide in case of aluminum prepared by electrolysis. Observations at the time of jar-test disclose that, in case of the electrolyzed aluminum in the present example, the time required for the formation of flock is shortened, and the size of flock is large as compared with others.

Example 2 By employing an electrolyzer such as shown in FIG. 2 (which was provided with electrode plates measuring 30cm X 30cm X 0.6cm and employing iron as the cathode) filled with a mixture solution of 0.2% NaCl and 0.2% HCl, and applying such polarity as shown in the same drawing and a current density of 16.7 mAlcm the electrolysis was carried out. Even after repeated electrolyses conducted at the rate of one cycle per 2 hours for more than 20 days, there was observed no adhesion of scale at all, and it was proved to endure a long-period operation at 130 percent current efficien- Example 3 Excepting employment of an electrolyzer such as shown in FIG. 3 or FIG. 4 in lieu of the electrolyzer of Example 2, the electrolysis was repeated in the same manner as described in Example 2. But, there was observed no formation of scale at all, and a long-period operation like the preceding example was carried out without trouble at all.

Example 4 Aldm the electrolysis was carried out for a total period of 1 15 hours. The result of said electrolysis is as shown in Table 3.

Electro- Weight of electrode (gr) Coefficient lytic Electbefore after of rode electrolysis electrolysis Utilization room l-a 1495 199 86.3 A

l-b M 220 85.2

l-c 1495 296 80.2 C

It is obvious from Table 3 as above, that each aluminum anode plate demonstrates almost uniform dissolution and there is no leak of current. Further, there was observed no adhesion of scale onto any anode plate.

What is claimed is:

l. A method of preparing an aqueous solution of an aluminum-containing coagulating agent for use in water treatment, which comprises the steps of subjecting an aqueous electrolyte solution to electrolysis between an aluminum anode and a cathode to effect dissolving of aluminum from the anode into the solution and the formation of an aqueous solution of aluminum-containing coagulating agent, said electrolyte solution at the start of said electrolysis consisting essentially of an aqueous solution of a mixture of (l) a strongly electrolytic salt selected from the group consisting of sodium chloride and sodium sulfate and (2) a strong acid selected from the group consisting of hydrochloric and sulfuric acids, the concentration of trolysis is effected by disposing aluminum anodes on both sides of a cathode consisting of a conductive material other than aluminum.

4. A method as defined in claim 3, wherein the cathode consists of iron.

5. A method as defined in claim 1, wherein the spring between the anode and the cathode is in the range of 10 to 40 mm, and the electrolysis is effected at a voltage in the range of 2 to 30V and a current density in the range of l to 2 A/dm. 

1. A method of preparing an aqueous solution of an aluminum-containing coagulating agent for use in water treatment, which comprises the steps of subjecting an aqueous electrolyte solution to electrolysis between an aluminum anode and a cathode to effect dissolving of aluminum from the anode into the solution and the formation of an aqueous solution of aluminum-containing coagulating agent, said electrolyte solution at the start of said electrolysis consisting essentially of an aqueous solution of a mixture of (1) a strongly electrolytic salt selected from the group consisting of sodium chloride and sodium sulfate and (2) a strong acid selected from the group consisting of hydrochloric and sulfuric acids, the concentration of said electrolytic salt in said electrolyte solution being in the range of 0.1- 0.5 wt. % and the concentration of said strong acid in said electrolyte solution being in the range of 0.05- 1 wt. %, based on the weight of the solution, respectively, and recovering said electrolyzed solution containing said dissolved aluminum-containing coagulating agent.
 2. A method as defined in claim 1, wherein said electrolytic salt is sodium chloride and said strong acid is hydrochloric acid.
 3. A method as defined in claim 1, wherein said electrolysis is effected by disposing aluminum anodes on both sides of a cathode consisting of a conductive material other than aluminum.
 4. A method as defined in claim 3, wherein the cathode consists of iron. 